1. Field of the Invention
The present invention relates to a transfer mechanism, as well as system and machine that incorporate the transfer mechanism, that transfers an image forming substance from one surface to another surface. More particularly, the invention relates to copying machines, printers, facsimile machines and similar image forming apparatuses that include an intermediate transfer element for transferring an image, and in particular, a color image as part of an image forming process.
2. Discussion of the Background
In the imaging art, there has been proposed a system wherein a transfer electrode, for example, a transfer roller, having a voltage applied thereto is held in contact with an image carrier in order to transfer a toner image from the image carrier to a recording medium. This kind of transfer system is desirable from an environmental and energy saving standpoint, primarily because the system does not rely on electron discharge, and thus produces a minimum of ozone and saves power.
A transfer roller that is frequently used as such a transfer electrode is referred to herein as a type A transfer roller and has a conductive core, or shaft, and a conductive layer formed on the shaft. The conductive layer is made from conductive fine grains, for example, carbon black or metallic grains, including titanium oxide or tin oxide, which may be dispersed in an insulating material, for example, EPDM (Ethylene propylene diene copolymer) silicon rubber.
By mixing a large amount of conductive grains, the type A transfer roller obtains a predetermined electric resistance value. However, due to difficulty in uniformly distributing the grains, the electric resistance value is less than perfectly uniform over the type A transfer roller. Consequently, the electric resistance of the type A transfer roller varies with the voltage applied thereto. Assuming the transfer roller is a type A roller, the applied voltage or current noticeably changes based on Ohm's law (E=IR), and this change adversely affects the image transfer characteristic of the device that uses the transfer roller and causes unsatisfactory image transfer operations.
To solve the above-described problems, a transfer electrode has been proposed, configured as a transfer roller, that has the conductive layer made of EPDM silicon rubber to which is added various kinds of metal ion salts, surface active agents or similar ionic agents. These additives help to reduce the dependency on the resistivity on the material on the applied voltage. However, as presently recognized, a problem that remains is that the characteristics of the material used for the roller are susceptible to the environment, particularly humidity, since the metal ion salts, the surface active agents or similar ionic agents absorb water. As a consequence, the electric resistance of the material changes depending on the environment.
Examples of devices where the electrical resistance is susceptible to environmental conditions is the device described in Japanese Laid-Open Patent Publication No. 8-220900, which describes a conductive roller produced by altering ion conductivity by incorporating a tetra butyl ammonium salt with urethane foam. Further, Japanese Laid-Open Patent Publication No. 08-328351 describes a conductive roller produced by adding ionic conductive material with the conductive base material by incorporating a NBR (acrylonitrile butadiene copolymer) rubber. Japanese Laid-Open Patent Publication No. 8-63014 discloses a conductive roller produced by mixing the conductive filler with rubber having a specific volumetric resistance. However, producing such conductive rollers in a cost effective manner is a challenge, and the incentive for overcoming this challenge is tempered by the relatively narrow characteristic transfer limits, as will be discussed, associated with such rollers.
Using such intermediate transfer elements in a color image forming apparatus presents other problems. For example, in a color image forming apparatus, separate toner images of each of the color components are formed on a photoconductive element in separate operations. Subsequently, the color toner images are transferred as separate toner images to the intermediate transfer element and later transferred to a recording medium, where the separate color images are made to be superimposed on one another on the recording medium so as to make a composite color image. In this situation, an image reproducibility problem arises when a type A transfer roller is used. In particular, reproducibility of a color, a part of a small amount of toner deposition on the intermediate transfer element, for example, mono-color toner (yellow, magenta, cyan or black) and a part of a large amount of toner deposition on the intermediate transfer element, full-color toner (yellow, magenta, cyan and black) becomes noticeably worse. The cause of the above-described reproducibility problem is not total clear, but the present inventor has made several observations that help to better characterize the problem and subsequently mitigate the problem.
First of all, an appropriate transfer efficiency depends upon a charge density established by an applied current. Assume that an electric resistance of the transfer roller differs between a part that will transfer a portion of the image having a small amount of toner to another part that will transfer another portion of the image having a large amount of toner. Under these conditions, the applied voltage will noticeably change across the transfer roller, and consequently, the efficiency of toner transfer from the intermediate transfer element to the recording medium may be adversely influenced by the combination of spatially variant toner amount-and applied voltage, which themselves are influenced by the image to be printed and the lack of resistance uniformity on the transfer roller.
When using the type A transfer roller, its electric resistance distribution noticeably changes, thereby the current which is applied from the transfer roller to the intermediate transfer noticeably changes for one image. Consequently, the charge density established by the applied current, so as to obtain the appropriate transfer efficiency, noticeably differs between the respective parts of the image. This difference is significant in the case of forming color images because the amount of toner actually deposited for the separate uni-color images varies substantially. A smallest amount of deposited toner occurs, for instance, when a uni-color image is printed with a low gray scale measurement and a highest amount of deposited toner occurs for an image having a high gray scale measurement and 4 overlapping/superimposed colors. Moreover, when the range of toner deposition varies greatly, the non-uniform charge distribution effect of the type A roller on image quality becomes noticeable and significant. Once again, the source of this problem may be attributable to the non-uniformity of the type A roller resistance and associated charge distribution, particularly in a color image forming operation,
Further explaining the problem, when using the transfer roller of type A in a color image forming apparatus that selectably places the roller in contact with the intermediate transfer element, an unsatisfactory image transfer of the color toner image from the intermediate transfer element to a leading edge of the recording medium is observed. The unsatisfactory image transfer is referred to as having a so-called transfer hollow. In case of the color image forming apparatus including the intermediate transfer element, the transfer roller is separated from and moved to contact the intermediate transfer element. When contacting the intermediate transfer element, a part of the transfer roller is compressed and deformed at a transfer nip portion where the intermediate transfer element and the transfer roller contact one another. As a consequence, an electric resistance of the compressed and deformed part of the transfer roller decreases.
The decrease of electric resistance is presumably due to the fact that when the roller is compressed, it is easier to move electrons between dispersed conductive fine grains and thus the current between the transfer roller to the intermediate element noticeably increases. Provided that the current is returned to normal after an appointed time by using a constant current power source, minimal harm is done. However, a performance problem manifests itself in that a transfer hollow occurs at a leading edge of the recording medium which corresponds to when the surge of current was present.